At baseline participants will have evaluations to determine bone mineral content and bone mineral density (Dual X-ray Absorptiometry and Qualitative Computed Tomography) and a blood sample collected to measure biomarkers of bone turnover, hormonal status and Vitamin D metabolism. Anthropometrics and tanner stage will be obtained at baseline, 3 months, 6 months, 9 and 12 months. A physical activity monitor will be worn for a 7 day period and a food frequency questionnaire will be completed. Tibial length will be measured. Participants have a blood sample collected either at home or at St. Jude Children's Research Hospital (SJCRH) every 3 months during the study to measure biomarkers of bone turnover, hormonal status and Vitamin D metabolism, anthropometrics and tanner stage, physical activity monitor and food frequency questionnaire. All measurements and evaluations required at baseline will be repeated at the completion of study visit.